Method for finding starting bit of reference frames for an alternating-parity reference channel

ABSTRACT

The present invention discloses a method for locating the reference frames of the reference lane on the transmitting data bus. The present invention addresses this object by disclosing a method whereby the relationship between the size of the reference frame transmitted over the reference lane and the width of the data bus is such that the reference frame is bit-shifted automatically until it is aligned with the data bus.

CROSS-REFERENCE TO RELATED APPLICATIONS

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STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

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REFERENCE TO A SEQUENCE LISTING, A TABLE, OR A COMPUTER PROGRAM LISTING COMPACT DISC APPENDIX

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BACKGROUND OF THE INVENTION

In computing, a data bus is simply a device which facilitates the transfer of data between data processing elements. In telecommunications applications, data busses can transfer data between separate elements on a board or even between specific instantiations of logic on a single chip, like an FPGA or an ASIC. A common data bus in these applications is the interface between serializing/deserializing (SERDES) devices and a data framer. In the interests of interoperability, the interface is typically instantiated in accord with the SERDES Framer Interface (SFI) implementation agreements published by the Optical Internetworking Forum (OIF). The latest of these agreements, rather than addressing a particular rate of data transfer, is directed towards the needs of future networks which will operate at rates beyond 40G. The 40G SFI standard, SFI-5, features four data transmission lanes as well as a fifth lane for the transmission of deskew data for the data frames being transmitted across the bus. Based on this architecture, the latest OIF standard details the implementation of an SFI bus that can incorporate 4-20 data lanes with a single deskew lane, and theorizes the implementation of even larger bus sizes beyond 20 lanes for future applications at speeds which remain in the realm of theory.

This latest scalable bus implementation is known as SFI-S. The OIF's SFI-S implementation is an expansion upon the concepts of the SFI-5 and SFI-4 standards. At its smallest size (4+1 lanes), SFI-5 closely resembles SFI-5, although some differences exist in SFI-S's deskew lane. The core concept of the OIF's SFI standards is a single 10 Gb/s data transmission lane with an accompanying deskew lane (SFI-4). Successive iterations of the standard offer increasingly larger numbers of data lanes transmitting multiplexed data frames. As these architectures have become more complex and consequently faster, the issue of skew has become more important: higher data rates mean greater sensitivity to timing errors.

As indicated above, the SFI standards address the issue of skew by including a deskew, or reference, lane in the design of the data bus. The deskew lane transmits a reference frame which allows deskew elements in the SERDES or framer to measure skew in the incoming data, and adjust the transmission of the data accordingly. The transmitted reference lane consists of a number of five-bit elements which are themselves comprised of four samples taken from the data lanes followed by a parity of those four bits. The parity bits may be even or odd, dependent upon their position in the reference frame. The SFI-S implementation agreement states that every reference frame must begin with an even parity element and end with an odd parity element. Any additional elements required alternate even and odd parity, but always begin with an even parity element. Thus, a four-element frame is defined as even, even, odd, odd. The number of required elements is dependent upon the number of data lanes; the maximum number of data lanes supported by a reference lane is n×4,where n is the number of elements, bearing in mind that there must be at least 2 elements per frame. Thus, 4-8 data lanes require a 2-element reference frame, 9-12 lanes require a 3-element reference frame, 13-16 lanes require a 4-element reference frame, and 17-20 lanes require a 5-element reference frame. As noted above, each of these elements are composed of four bit samples from the data lanes. The sampling method is defined in the implementation agreement:

-   -   The bit samples of the reference frame shall be filled, starting         at the highest number data lane RX/TXDATA(n−1), which is filed         into the first sample position of the even parity element at the         beginning of the frame. RX/TXDATA(n−2) is filled into the second         sample position and so on, until RX/TXDATA(0) is filled in. If         the RX/TXDATA(0) sample does not land in the last sample         position of the frame ending odd element, the remaining bit         samples are filled again, starting from RX/TXDATA(n−1). (SFI-S)

This sampling method, for a ten-lane configuration, is illustrated in FIG. 1. The deskew operation is dependent upon the proper construction of reference frames; it is therefore essential that the reference frames begin with the correct bits, so that the lanes of the data bus may be correctly deskewed. It is possible, however, that sampling may begin with the incorrect bit, due to a timing error or skew. If such an event occurs, it is necessary to spend additional on-board resources to hunt for the correct starting bit of the reference frames. A common means of rectifying this issue is to employ a bit-shifting element to alter the starting position of bit sampling for the reference lane. Employing such a bit-shifting element requires the use of additional resources, even if an already-present shifting element must be reconfigured to alter the position of the sampled bits. Thus it is necessary to employ a method which can address the issue of a misaligned reference lane without further requiring the use of additional on-board resources.

BRIEF SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method for locating the reference frames of the reference lane on the transmitting data bus. The present invention addresses this object by disclosing a method whereby the relationship between the size of the reference frame transmitted over the reference lane and the width of the data bus is such that the reference frame is bit-shifted automatically until it is aligned with the data bus.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of the composition of an SFI-S reference frame for a ten-lane data bus.

FIG. 2 is an illustration of an embodiment of the present invention, wherein data is transmitted over a ten lane, 64-bit wide bus.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

The present invention discloses a method for finding the starting bit of the reference frames of the reference lane on a data bus. The invention comprises transmitting a reference frame over the reference lane, wherein the reference frame is comprised of a plurality of five-bit elements, wherein each of those five-bit elements is comprised of four bit samples from the data lanes of the data bus, followed by the parity of those four bits; as the reference frame is being transmitted, if the relationship between width of the reference frame and the width of the data bus is such that the reference frame cannot fit evenly within the confines of the data bus, then the difference in bits between the width of the reference frame and the width of the data bus allows the starting bit of the reference frames to cycle through each possible bit position until the correct starting bit is found, without employing an external bit-shifting element.

FIG. 2 is an illustration of an embodiment of the present invention, featuring an exploded view of a 64-bit data bus over which ten lanes of data are being transmitted. In accord with the SFI-S Implementation Agreement, a ten-lane embodiment requires a three-element reference frame, wherein each element is five bits wide, and thus creating a fifteen-bit reference frame. The marked bits in each data lane correspond to a sampled bit in the reference frame (not shown). Thus, bits 1.0-3 represent the first four bits of the first element of the first reference frame, bits 2.5-8 represent the second four bits of the second element of the second reference frame, bits 5.0-3 represent the first four bits of the first element of the fifth reference frame, and so on. The sixteen marked bits at the bottom of the figure represent bit “positions” 01-16 within the data bus, imposed upon the bus in the figure for the sake of clarity in offering a detailed description of this particular embodiment of the invention. In a like manner, the data bus has been evenly divided in the figure into four 16-bit wide segments for the sake of clarity. It should be noted that this illustrative embodiment is provided for exemplary purposes only and is not intended to limit the scope of the invention.

As noted above, the ten lane configuration requires a fifteen-bit reference frame transmitted over the reference channel. The reference channel takes samples from the data being transmitted over the 64-bit data bus. The relationship in size between the reference frame and the data bus is most easily represented as a quotient between the two; thus, when the 64 bit bus is divided by the 15-bit reference frame, we find that the result shows four whole reference frames for every 64 bits, with a remainder of four bits left over. So, in this embodiment, for every single reference frame transmitted there is a single leftover bit.

This single bit differential between the reference frames and the width of the data bus ensures that, if not locked, the reference frame shifts its position in relation to the data bus with every reference frame, without the use of an external element. This shift in position is also illustrated in FIG. 2.

In FIG. 2, the first bit of the first reference frame (1.0) is sampled in position 01 in relation to the data bus. Because of the single bit differential noted above, however, the first bit of the second reference frame (2.0), has shifted its position to 16. With each successive reference frame, the position of that frame's first bit shifts by one. Thus, the third frame begins at position 15, the fourth at position 14, and the fifth at position 13. We can infer that the reference frame will continue to shift into a different position until it starts at 01 again, 16 clock cycles after startup; at least one of these positions will be aligned with the data bus, and so a maximum of 16 clock cycles are required to find the correct starting bit for the reference frames on the reference lane within the transmitted data, so that deskew can occur on the receive side. Once the starting bit is found, the reference channel bit position is locked so that the position of the reference frame no longer shifts; in this way, the bits of the reference frames are sampled correctly by taking advantage of the bit differential between the size of a reference frame and the width of the bus, without the use of an external bit-shifting element. 

What is claimed is:
 1. A method for finding the correct starting bit of each a plurality of reference frames on a reference channel for a data bus which is n bits wide, comprising: a. transmitting a plurality of data over a data bus, wherein said data bus is comprised of a plurality of data transmission channels in parallel and a reference channel; b. sampling said data transmission channels at successive intervals to create a plurality of samples; c. transmitting a plurality of reference frames over said reference channel, wherein each of said plurality of reference frames is comprised of said plurality of samples, and wherein each of said plurality of frames is m bits wide and begins with a starting bit, whereby a quotient of n/m is equal to x with a remainder of y, wherein said remainder indicates a multiple of a number of bits that said starting bit of each successive reference frame will shift in position in relation to said data bus; d. waiting for a maximum of m+1 reference frames to be transmitted, wherein said starting bit of each of said maximum of m+1 reference frames shifts through one of a maximum of m+1 possible positions in relation to said data bus; e. locking said reference channel when one of said starting bits of said maximum of m+1 reference frames is transmitted in one of said maximum of m+1 possible positions, wherein said one of said maximum of m+1 possible positions is aligned with said data bus.
 2. The method of claim 1, wherein n is equal to
 64. 3. The method of claim 1, wherein said data bus is comprised of 10 data transmission channels.
 4. The method of claim 1, wherein said reference channel is an alternating-parity reference channel.
 5. The method of claim 3, wherein each of said reference frames consists of three five-bit elements.
 6. The method of claim 5, wherein each five-bit element consists of four bit samples from said data transmission channels followed by a parity of those four bits.
 7. The method of claim 7, wherein each successive reference frame shifts one bit in position in relation to said data bus.
 8. The method of claim 1, wherein said maximum number of reference frames is
 16. 